The present invention relates to DNA sequences which lead to the formation of polyfructans (levans), as well as a process for preparing transgenic plants using plasmids on which these DNA sequences are located.
High molecular weight, water soluble, linear polymers, for example those based on polyacrylates or polymethacrylates, are products of mineral oils and have many important uses. In particular their properties in increasing the viscosity of aqueous systems, in suspending or sedimentation acceleration and complexing are especially valuable from the technical viewpoint. These products are also used in exceptionally large amounts in super absorbers for water binding and in water dilutable lacquers. In spite of the outstanding positive properties, because such products are difficult to dispose of, their use is increasingly coming under criticism because they are not biodegradable.
Alternatives based on recyclable raw materials, especially starches and cellulose, because of the macromolecular structure of these polysaccharides, have been shown to have limited value. As a replacement for non-biodegradable chemically derived polymers, a number of derivatised high polymeric polysaccharides have been considered. Until now, such polysaccharides could only be obtained biotechnologically via suitable fermentation and transglycosidation processes. The products obtained in this way, such as dextrans and polyfructans (levans) are not competitive as raw materials for mass production.
Polyfructans are found in a number of monocotyledonous and dicotyledonous higher plants, in green algae as well as in a number of gram positive and gram negative bacteria (Meier and Reid, (1982) Encyclopedia of Plant Physiology, New Series, 13A 418-471). The role of fructans for the plant development and plant growth is not fully understood. Functions of the fructans that have been proposed are as a protectant against freezing at low temperatures, as alternative carbohydrate stores which limit starch biosynthesis, as well as applied intermediary stores for photoassimilates which are situated in the stems of grasses shortly before their transfer into the seeds.
All fructans contain, as starter molecule for the polymerisation reaction, a molecule of sucrose (glucose-fructose) to which fructose polymers are added.
Depending on the coupling of the fructose molecule, fructans of-plant origin can be classified into four classes (Meier and Reid (1982), Encyclopedia of Plant Physiology, New Series, 13A, 418-471):
a) (2-1) coupled xcex2 D-fructans (inulin type)
b) (2-6) coupled xcex2-D-fructans (phlein or levan type)
c) highly branched fructans with a mixture of 2-1 and 2-6 couplings.
d) (2-1) coupled xcex2-D-fructans, which in contrast to the types under a-c, are added completely from fructose residues of polymerisation both from glucose and also from fructose residues from polyfructose residues (neokestose type).
Fructans of bacterial origin correspond either to the levan or to the inulin type (Carlsson (1970) Caries Research 4, 97-113) and Dedonder (1966) Methods Enzymology 8, 500-505).
Experiments on the biosynthesis of fructans in plants and bacteria lead one to conclude that the biosynthesis proceeds by various routes. Bacterial and plant fructans are further distinguished, not particularly in their primary structure but mainly in their molecular weight. Thus, fructans isolated from plants have been shown to have molecular weights of between 5000 and 50,000 d (Pollock and Chatterton (1988) in: The Biochemistry of Plants 14, 109-140), while fructans isolated from bacteria, molecular weights of up to 2,000,000 d have been described (Clarke et al (1991) in: Carbohydrates as Organic Raw Materials, VCH Weinheim, 169-182).
Various microorganisms from the group of Bacillus spp as well as Streptococcus spp produce polyfructoses in which both fructans of the levan type and fructans of the inulin type have been described (Carlsson (1 970) Caries Research 4, 97-113 and Dedonder (1966) Methods Enzymology 8, 500-505).
Experiments on biosynthesis pathways have made it clear that, in comparison to biosynthesis pathways in higher plants, there is a simpler pattern and a sharing of only one enzyme. This enzyme with the trivial name levan sucrase is a transfructosylase (sucrose:.xcex2-D-fructosyl transferase, E.C.2.4.1.10.), which catalyzes the following reaction:
sucrose+acceptorglucose+fructosyl acceptor
Representative acceptors are water, alcohol, sugar or polyfructoses. The hypothesis that only one enzyme catalyses this reaction, depends on the one hand on the examination of the protein chemically purified enzyme, and on the other, to the fact that the gene for levan sucrase has been isolated from various Bacillus spp. as well as from a Streptococcus spp. and after transfer into E. coli leads to the formation of levan in E. coli (Gay et al (1983) J. Bacteriology 153, 1424-1431 and Sato et al. (1986) Infection and Immunity 52, 166-170).
Until now, genes for levan sucrase from Bacillus amyloliquefaciens (Tang et al. (1990) Gene 96, 89-93) and Bacillus subtilis (Steinmetz et al. (1985) Mol. Gen. Genetics 200, 220-228), have been described, and demonstrate relatively high homology with each other and both of which catalyze the synthesis of fructans of the levan type. Further, a fructosyl transferase from Streptococcus mutans (Shiroza et al. (1988) J. Bacteriology 170, 810-816) has been described. This shows little homology to either levan sucrases from Bacillus spp.. The fructan formed in Streptococcus mutans is of the inulin type.
In WO 89/12386, there is described the possibility of producing carbohydrate polymers such as dextran or levan in transgenic plants, especially in the fruit of transgenic plants. To prepare these plants, the use of levan sucrases from Aerobacter levanicum, Streptococcus salivarius and Bacillus subtilis and the use of dextran sucrases from Leuconostoc mesenteroides have been described.
Further, the construction of chimeric genes is described which may be suitable for the expression of the levan sucrase from Bacillus subtilis as well as the dextran sucrase fom Leuconostoc mesenteroides in transgenic plants. Also described is the preparation of transgenic plants containing these constructs. Further, the preparation of transgenic plants that contain these constructs are described. Whether polyfructans can actually be produced by the described process is not known.
There is also described a series of processes for modifying the carbohydrate concentration and/or concentrating carbohydrates in transgenic plants by means of biotechnological methods. Thus, in view of the fact that increasing of the starch concentration and the modification of the starch in physical and chemical respects is already known, then a modification of the carbohydrate content of potato plants by raising or lowering the ADP-glucose-pyrophosphorylase activity can be achieved (EP 455 316).
From EP 442 592 it is further known that a modification of the distribution of photoassimilates by means of cytosolic and apoplastic invertase is possible and that the yield as well as the drought and frost resistance of potato plants can be modified through the expression of a heterologous pyrophosphatase gene in potato plants.
In order to adapt the physico-chemical parameters of raw materials which are increasingly being used, such as polysaccharides, to the requirements of the chemical industry, as well as to minimize the costs of obtaining these products, processes for the preparation of transgenic plants have to be developed which lead in comparison with known processes to better, higher yielding plants.
It has now been surprisingly found that the DNA sequence of the levan sucrase from a gram-negative bacterium of the species Erwinia amylovora with the nucleotide sequence (Seq-ID NO 1):
makes possible the preparation of large amounts of polyfructans (levans) in transgenic plants, which decisively meet the needs of the chemical industry in respect of recyclable raw materials.
By integration of a DNA sequence in a plant genome, on which the above given DNA sequence is located, the polyfructan (levan) expression in plants, especially in leaves and tubers is made possible. The levan sucrase of the invention shows, at the DNA level, no significant homology to the known levan sucrases.
The invention further provides a process for the preparation of transgenic plants with polyfructan (levan) expression in leaves and tubers that comprises the following steps:
(a) preparation of a DNA sequence with the following partial sequences:
i) a promoter which is active in plants and ensures formation of an RNA in the intended target tissues or target cells,
ii) a DNA sequence of a levan sucrase, and
iii) a 3xe2x80x2-non-translated sequence, which in plant cells leads to the termination of the transcription as well as the addition of poly A residues to the 3xe2x80x2-end of the RNA,
(b) transfer and integration of the DNA sequence in the plant genome of a recombinant double stranded DNA molecule from plant cells using a plasmid, and
(c) regeneration of intact whole plants from the transformed plant cells.
The levan sucrase obtained in process step (a,) ii) preferably shows the nucleotide sequence noted under sequence ID No 1.
The levan sucrase catalyses the following reaction:
Sucrose-(fructose)n+sucrosesucrose-(fructose)n+1+glucose.
Using this process in principle, all plants can be modified in respect to a polyfructan (levan) expression, preferably crops such as maize, rice, wheat, barley, sugar beet, sugar cane, tobacco and potatoes.
In process step (b), in principle, all plasmids can be used which have the DNA sequence given under sequence ID No 1. Preferably used are plasmid p35s-CW-LEV (DS3.M) 7186), plasmid P35s-CY-LEV (DSM 7187) or plasmid P33-CW-LEV (DSM 7188).
Since sucrose represents the substrate for the levan sucrase, the production of polyfructans is especially advantageous in those organs that store large amounts of sucrose. Such organs are for example, the roots of sugar beet or the stems of sugar cane. It is especially useful in genetically modified potatoes, which store sucrose in their tubers, through the blocking of starch biosynthesis.
Biosynthesis of sucrose takes place in the cytosol, while in contrast, storage is in the vacuole. During transport into the storage tissues of a sugar beet or potato or into the endosperm of seeds, the sucrose must cross the intercellular space. In the production of polyfructans, all three cell compartments are suitable, i.e. cytosol, vacuole and intercellular space.
The coding sequence of the levan sucrase of the nucleotide sequence ID No 1 can be provided with a promoter that ensures the transcription occurs in a specified order and which is coupled in sense orientation (3xe2x80x2-end of the promoter to the 5xe2x80x2-end of the coding sequence) on to the coding sequence which codes for the enzyme to be formed. The termination signal, which determines the termination of the mRNA synthesis, is adhered to the 3xe2x80x2-end of the coding sequence. In order to direct the enzyme which is expressed in specified sub-cellular compartments such as chloroplasts, amyloplasts, mitochondria, vacuoles, cytosol or intercellular space, a so-called signal sequence or a transit peptide coding sequence can be positioned between the promoter and the coding sequence. This sequence must be in the same reading frame as the coding sequence of the enzyme.
For the introduction of the DNA sequence of the invention in higher plants, a large number of cloning vectors are available which contain a replication signal for E. coli and a marker which allows a selection of the transformed cells. Examples of vectors are pBR 322, pUC-series, M13 mp-series, pACYC 184; EMBL 3 etc.. According to the introduction method of the desired gene in the plant, other DNA sequences may be suitable. Should the Ti-or Ri-plasmid be used, e.g. for the transformation of the plant cell, then at least the right boundary, often however both the right and left boundaries of the Ti-and Ri-Plasmid T-DNA, is attached, as a flanking region, to the gene being introduced. The use of T-DNA for the transformation of plants cells has been intensively researched and is well described in EP 120 516; Hoekama, In: The Binary Plant Vector System, Offset-drukkerij Kanters B.V. Alblasserdam, (1985), Chapter V; Fraley, et al., Crit. Rev. Plant Sci., 4:1-46 and An et al. (1 985) EMBO J. 4: 277-287. Once the introduced DNA is integrated in the genome, it is as a rule stable there and remains also in the offspring of the original transformed cells. It normally contains a selection marker, which induces resistance in the transformed plant cells against a biocide or antibiotic such as kanamycin, G 418, bleomycin, hygromycin or phosphinotricin etc. The individual marker employed should therefore allow the selection of transformed cells from cells that lack the introduced DNA.
For the introduction of DNA into a plant, besides transformation using Agrobacteria, there are many other techniques available. These techniques include the fusion of protoplasts, microinjection of DNA and electroporation, as well as ballistic methods and virus infection. From the transformed plant material, whole plants can be regenerated in a suitable medium that contains antibiotics or biocides for the selection. The resulting plants can then be tested for the presence of introduced DNA. No special demands are placed on the plasmids in injection and electroporation. Simple plasmids, such as e.g. pUC-derivatives can be used. Should however whole plants be regenerated from such transformed cells the presence of a selectable marker gene is necessary. The transformed cells grow within the plants in the usual manner (see also McCormick et al. (1986) Plant Cell Reports 5: 81-84). These plants can be grown normally and crossed with plants that possess the same transformed genes or different. The resulting hybrid individuals have the corresponding phenotypical properties.
Deposits
The following plasmids were deposited at the Deutschen Sammlung von Mikroorganismen (DSM) in Braunschweig, Germany on the 16.07.1992 (deposit number):
Plasmid p35s-CW-LEV (DSM 7186)
Plasmid p35s-CY-LEV (DSM 7187)
Plasmid p33-CW-LEV (DSM 7188)